Cd(II) speciation in alginate gels.
Polysaccharides, such as those occurring in cell walls and biofilms, play an important role in metal speciation in natural aqueous systems. This work describes the speciation of Cd(II) in alginate gels chosen as a model system for biogels. The gels are formed by bridging calcium ions at junction zones present along adjacent homopolymeric guluronic acid chain sequences. The free Cd2+ concentration in the gel phase is measured by a novel in situ microelectrode voltammetric technique that monitors the electroactive probe cation Cd2+ by its reduction at a Au-amalgam microelectrode. In situ voltammetric measurement coupled with total Ca(II) and Cd(II) determinations, as well as potentiometric titration, permits the full reconstruction of the charging environment and the cation binding forthe gel phase. Three independent combinations of measuring and modeling the charged gel layer thereby permit accurate prediction of the Donnan potential, psiD, and the Donnan enrichment coefficient, piD. At an ionic strength of 10 mM, Donnan potentials in the gel ranged from approximately -10 to -20 mV, corresponding to an enhancement in the level of free Cd2+ ions in the gel phase relative to the bulk solution by a factor of approximately 3. In contrast, the total level of Cd(II) was found to be enhanced by a factor of approximately 60, resulting predominantly from the specific binding of the Cd bythe uronic acids of the alginate gel. These results emphasize that large differences in Cd(II) speciation can arise due to the combination of specific and electrostatic modes of binding. The results of this speciation analysis, for charged biological gels, have important consequences for mechanistic interpretation of metal biouptake processes involved in complex media.
Davis
Thomas A
TA
Laboratory of Physical Chemistry and Colloid Science, Wageningen University, P.O. Box 8038, 6700 EK Wageningen, The Netherlands. ta.davis@umontreal.ca
Kalis
Erwin J J
EJ
Pinheiro
Jose Paulo
JP
Town
Raewyn M
RM
van Leeuwen
Herman P
HP
D000464:Alginates / Q000737:chemistry*; D001682:Biological Availability; D002104:Cadmium / Q000737:chemistry*; D002118:Calcium; D005782:Gels / Q000737:chemistry; D020723:Glucuronic Acid / Q000737:chemistry; D006603:Hexuronic Acids / Q000737:chemistry; D007700:Kinetics; D017186:Titrimetry
D000464:Alginates; D005782:Gels; D006603:Hexuronic Acids; D002104:Cadmium; D020723:Glucuronic Acid; D002118:Calcium